Automotive manufacturers and the National Highway Transportation Safety Association are investigating methods to disable vehicle air bags in situations where they may cause more harm than good. Typically, airbags have been developed to deploy with enough force to restrain a 175 lb. adult in a high velocity crash. Deployment of the same air bags when children are seat occupants may cause serious injury due to the force generated upon inflation of the bag.
As a result, seat weight sensors and seatbelt tension systems are being developed in an attempt to determine whether a seat occupant is a child. Such systems should identify when the occupant is small, or even when a child is in a rear facing infant seat, a forward facing child seat or a booster seat. Occupant weight measurement when a child seat is present is further complicated by the downward force applied to the child seat by the tension of a seat belt. When a child seat is strapped tightly, the seat belt forces the child seat into the vehicle seat and can often artificially increase the measured weight, which may lead to air bag deployment when children or infants are present in the seat.
Tension measurement mechanisms have been incorporated in the buckle of a seatbelt. In one embodiment, a sliding buckle is biased with a spring. When the belt is under heavy tension, the buckle pulls forward to control a switch that provides feedback to a vehicle processor.
Additionally, it is known to detect seat belt tension by attaching a spring steel bend sensor to one side of the belt. When belt tension increases, a resistance change occurs in the sensor and the analog signal is converted to an approximate belt tension. However, field test indicates that such sensors tend to drift with temperature and, therefore require temperature compensation.
The aforementioned seat belt tension measurement methods suffer from a number of disadvantages. Initially, a great number of additional parts are required for seat belt retractors or buckle configurations thereby adding complexity and cost to vehicle assembly and providing considerable difficulty in retrofitting existing vehicles. Additionally, mechanical switches and bend sensor systems have a limited service life, thereby requiring periodic replacement or adjustment.
The present invention may be used to detect whether the seat belt is under high tension thereby indicating that an infant seat or another inanimate object is belted into the seat. The instant invention can be used in conjunction with a seat weight sensor to determine whether an airbag should be deployed for a given occupant. Additionally, the instant invention provides a continuous measurement of seat belt tension and may be used to provide a threshold level of detection where desired.